The present invention relates to water samplers of the type known as in situ pump samplers. The invention particularly relates to in situ pump samplers for trace substances in lake water or seawater, with a particular application to seawater.
The use of in situ concentration for various trace substances in seawater offers clear advantages over conventional methods requiring bulk water sample collection and shipboard extraction. For some substances of oceanographic, geochemical, or toxicological interest the degree of concentration required to ensure reasonable accuracy and precision of analysis necessitates the handling of very large volumes of sample. In the past, samplers have been built which collected thousands of liters of seawater and required twelve or more hours of station time for sampling and processing, in order to collect enough of some rare component for reliable analysis. For other components, large samples were needed in order to overcome the problem of large and variable background or blank values.
Until fairly recently, conventional methods of collection and concentration seemed adequate for most trace substances, since the analytical limits of detection exceeded apparent levels of contamination. However, a review of the earlier work reveals that in many instances contamination actually acquired during collection and extraction comprised the major part of the reported concentrations. For an even greater number of substances, more recent methods of analysis provide sensitivities which far exceed the capability of the most careful water sampling regime to deliver contamination-free samples.
An examination of the possible sources of contamination has provided insights useful in devising a more effective sampling strategy. The sources of contamination include the hydrographic wire carrying the sampling container and the sampling container itself. The early samplers, the Knudsen and Nansen bottles, used metal alloy valves requiring heavy application of stopcock grease, and were thus unfit for either trace metal or trace organic analyses. More recent samplers often contain both trace metals and plasticizers in the plastic used in their construction. The research vessel on station is surrounded by an aura of waste products through which the sampler must pass. Once the sample has been collected and brought on board for extraction, the usual hazards of contamination common to any laboratory are present, augmented by those special contaminants resulting from shipboard operations.
The oceanographic literature provides evidence of the considerable effort expended in attempts to reduce contamination. Hydrographic wires encapsulated in polymers, or made of materials such as Kevlar (Trade Mark) have been described, as have samplers made of noncontaminating materials or employing a large volume/ internal surface ratio. Remote sampling buoys have been utilized, and clean rooms have been fitted to some research vessels. While all of these strategies for avoiding sample contamination have met with some success, they suffer from the limitations of inconvenience, high cost, lack of versatility, and the need for large vessels with special winches or adequate deck space. At a time when the availability and expense of big ship operation will inevitably force the oceanographic community, outside of the large government laboratories to do much of its sample collecting from ships of opportunity, these do not appear to be useful strategies for the university or the small government laboratory.
In situ pumping offers a means of concentrating various trace substances from seawater while diminishing much of the potential for contamination inherent in water sample collection and shipboard extraction. Furthermore, where large samples are desired, the volume sampled is limited only by the capacity of the power source and the efficiency and capacity of the method of concentration.
For many chemical entities, the concentrator of choice is some variety of adsorption column. In recent years the chemical literature has overflowed with reports of newer and better column packings, with capabilities for concentrating an ever-increasing list of trace substances.
Some studies in which in situ pumping has been used have been described in the oceanographic literature. While the great potential of the method is apparent from these publications, the pumping systems used are invariably large, heavy, and are powered either from the surface or by specially prepared lead-acid storage batteries.
A pumping system powered from the surface requires a winch carrying conductor cable and equipped with slip rings. Such winches are not common either on ships of opportunity or on the smaller oceanographic research vessels. If a reasonable amount of power is needed, the system must either be limited to shallower depths or it must transmit a very large voltage at the surface in order to overcome the power loss in transmission.
The lead-acid storage batteries typically used for in situ pumping have proven effective but both inconvenient and hazardous. These batteries require a means of pressure equilibration, and provision must be made for degassing and for insulation of their terminals.
To obtain the most out of in situ pumping versality of the device must be considered. The pump sampler should be capable of pumping accurately whether large volume samples or small volume samples are involved. The system should be light and compact and should be of a nature such that more than one unit could be accommodated on a hydrographic wire in a single cast, so that depth profiles may be obtained, if desired, the system should be capable of activation and deactivation by a very simple contrivance and the device should be capable of alignment during operation to minimize the effects of sample contamination.